Relief valve



g- 21, 1956 H. u. GARRETT El AL 2,759,488

RELIEF VALVE 2 Sheets-Sheet 1 Filed June 16, 1951 Henry U. GarrelfINVENTORJ BY I A rromwsw Aug. 21, 1956 Filed June 16, 1951 u. GARRETT ElAL 2,759,488

RELIEF VALVE 2 Sheets-Sheet 2 ATTORNEYS United States Patent RELIEFVALVE Clifford M. Peters, Houston, Tex., Oil Tools, Inc., Houston, Tex.,a

Henry U. Garrett and assignors to Garrett corporation of Texas Thisinvention relates to improvements in pressure relief valves and refersmore particularly to that class of pressure relief valves of thedifferential piston type.

This invention is an improvement of the pressure responsive valve of ourcopending applications for United States Letters Patents, Serial Number74,875, filed February 7, 1949 (now Patent No. 2,552,892, issued May 15,1951), and Serial Number 164,872, filed May 29, 1950 (now Patent No.2,619,109, issued November 25, 1952).

It is common practice throughout industry, particularly in petroleumindustry, to vent a plurality of pressure relief valves into a commondischarge line or header in order to conduct any discharged relief fluidfrom the vicinity of the valves. In many instances the common dischargeline is quite long, leading to a flare or the like, so that when one ofthe relief valves discharges fluid thereinto, there is exerted aconsiderable back pressure on the other relief valves.

When the relief valves of such systems are of the differential pistontype, that is those valves wherein the seating member if urged intoseated position by application of the fluid pressure from the systembeing protected to a piston connected thereto, the piston having agreater effective pressure area than that of the seating member, a backpressure will be effective against the piston on the opposite sidethereof from the fluid pressure of the system being protected. Shouldthe back pressure on a valve exceed that of the system being protected,the valve will be opened even though the pressure in the system is belowthe set pressure of the relief valve. For example, when a system beingprotected by a relief valve is bled down to atmospheric pressure, it isnow necessary to blind off the relief valve in order to prevent it frombeing opened by the back pressure of another valve discharging into acommon header. Obviously, the same disadvantage is inherent in othertypes of relief systems wherein a back pressure is produced by any meanswhatsoever.

It is an object of this invention to provide a relief valve of thisgeneral class which will remain seated irrespective of a back pressureapplied thereto.

Another object of this invention is to provide a valve of this generalclass with means for preventing a back pressure from unseati-ng thesame.

Another object of this invention is to provide a valve of this generalclass in which means are provided for preventing a back pressure fromacting against the piston in opposition to the pressure of the systemprotected.

Yet another object of this invention is to provide a differential pistontype relief valve wherein there is provided a constant pressure chamberon the side of the piston opposite from that exposed to the pressure inthe system being protected whereby a back pressure applied to the valveis ineffective in causing it to open.

Other objects, advantages, and features of this .invention will beapparent to one skilled in the art upon consideration of the writtenspecification, the appended claims and the attached drawings; wherein:

Fig. 1 is a partial cross-sectional view of an illustrative embodimentof the relief valve of this invention;

Fig. 2 is a cross-sectional view of the pilot means for relieving thecontrol pressure within the main Valve;

Fig. 2A is a cross-sectional view of the pilot valve for repressuringthe main valve;

Fig. 3 illustrates one application of the valve of this invention; and

Fig. 4 is a cross-sectional View except that the valve element issition. V In the drawings, like characters of reference are usethroughout the several views to designate like parts.

Referring to Fig. 3 of the drawings, the reference numeral 1t)designates the relief valve of this invention which can be connected toa conduit or pipe line 11 of the pressure system being protected. Suchconduit can be the discharge line from an individual tank of a batteryof the same or the discharge line from compressors or any other systemwhose pressure is to be protected. Relief valves 10 can be connected toa common discharge header 13 by means of conduit stubs 14 so that thefluid relieved from the systems 12 can be conveyed through conduit 13 toa point remote therefrom, such as to a flare or the like.

Referring now to Fig. l, the relief valve comprises a casing 15 and abonnet 16. The casing is provided with an inlet or pressure relief port17 which can be connected to the system to be protected by means of aflange connection, and with an outlet port 18 adapted to be attached toa common discharge header such as conduit 13 by means of a suitableflange connection.

The interior of casing 15 includes a chamber 19 which is preferablycylindrical and in axial alignment with relief port 17.

A seat 20 is provided for relief port 17 and is adapted to receive valvemember 21 which engages the seat and is urged therefrom by the pressurewithin the system whose pressure is to be controlled.

A pressure means including a pressure responsive member is provided inthe casing and is adapted to urge the valve member to a seated positionwhen the pressure in the system protected is below a predeterminedvalue. Such means can comprise a piston 22 residing within the casingand separating chamber 19 from the relief port and outlet port. Piston22 has a sealing means 22a forming a sliding seal with the walls ofchamber 19 and is adapted to reciprocate therein toward and away fromseat 20. A power transmitting connection 23 is provided to connect thepiston to the valve member. In the interest of simplicity it ispreferred to form the piston and valve member as integral units.

In accordance with one aspect of this invention, there is provided aconstant pressure chamber open to the side of the pressure responsivemember opposite from that against which the pressure from the system tobe controlled acts. This constant pressure chamber ensures that thepressure actuating the pressure responsive member will not be partiallycanceled in effect by any back pressure applied to the valve. A suitablemeans for providing this chamber can comprise an inwardly extendingshoulder or flange 24 joined at its periphery to casing 15 and extendinginwardly to have a central opening therein through which powertransmitting connection 23 passes in slidable relationship.

A sliding seal between flange .24 and connection 23 can be provided byan O-ring 26 so as to prevent the passage of fluid from one side offlange 24 to the other. Thus, since piston 22 is likewise provided witha sliding of the valve of Fig. 1 shown in an open poseal, there will beprovided between piston 22 and flange 24 a constant pressure chamber 27which can be opened to the atmosphere through passage 28 or to any otherconstant pressure source.

Valve member 21 can be constructed by having a flange 29 extendingoutwardly from connection 23 and adapted to overlie and abut against theshoulder of seat 20. Disposed in a recess on the side of flange 29closest to the inlet port 17 is a sealing means 30, preferably ofrubber, which abuts against seat 26 to form a fluid tight seal therewithWhen the valve member is in seated position. The sealing means can beheld in place by a spider guide 31 having downwardly extending guidearms 31a sliding along the inner walls of inlet port 17 to guide thevalve member into proper position. A nut 32 is provided to hold guide 31in position.

The cylinder 19 and piston 22 have larger diameters and effectivepressure areas than those of seat 29 and valve member 21. Thus, it willbe apparent that when pressure within chamber 19 is equal to that withinthe system protected, the resultant force due to pressure acting againstthe piston and valve member will hold the valve member in a seatedposition.

A bleed means is provided for supplying pressure fluid from the systemprotected to chamber 19 when the valve member is seated. This means caninclude a central passage 33 through the valve member-piston unit. Meansis provided to close this passage when the valve member is in openposition, that is, when the pressure in chamber 19 has been relievedsufficiently to permit piston 22 to move towards the bonnet asillustrated in Fig. 4. The closing means is illustrated as including anelement or bolt 34 depending from bonnet 16 into the discharge end ofpassage 33a which is a continuation of passage 33 contained withinbushing 35. This bolt has a central passage 36 of a susbtantiallysmaller cross-sectional flow area than passage 33 so that it acts as aconstriction or choke. It has laterally extending end ports opening intochamber 19. Seal means, as for example, O-rings 38, are provided forsealing between bolt 34 and passage 33a on both sides of the lateral endports, when the piston-valve unit is in its Fig. 4 or open position.

The upper end of bushing 35 can be tapered to have walls divergingtoward the piston end of the unit so that O-rings 38 can be easilyreceived within passage 33a to form a seal.

Bolt 34 is suspended from bonnet 16 by a shoulder 40 formed thereon andreecived in a counterbore in the bonnet. A Washer 41 is placed aroundbolt 34 and the entire assembly held in place by a split ring 43situated in a groove in the bonnet.

It is believed to be obvious from the foregoing that when the valvemember is seated, as shown in Fig. 1, passages 33, 33a, and 36 willestablish a communication between the interior of chamber 19 and thepressure system protected so that such pressure can be exerted over theeffective pressure area of piston 22 to urge the valve member to aseated position. However, when the valve member is in an open position,as shown in Fig. 4, bolt 34 will be pushed into passage 33a so that thelateral ports from passage 35 and their adjacent seal rings will preventflow of fluid to chamber 19. It is to be noted that the capacity ofpassage 36 is so small that the fluid from the system protected slowlybleeds into chamber 19.

In order to unbalance the forces due to pressure acting on piston 22 andvalve member 21 upon occurrence of a predetermined maximum pressurewithin the system protected to thereby open the relief valve, a highpressure pilot relief means, designated generally by the numeral 45, isprovided for relieving the pressure within chamber 19. This pilot meansis of a self-closing snap-action type and preferably is of such type asw1ll remain open for a definite time period suflicient to provide reliefof the pressure within chamber 19.

Although a particular pilot relief valve means has been developed forthis purpose and is preferred, it is to be understood that other reliefvalves of this general class can be employed with reasonablysatisfactory results.

The preferred pilot valve controls the relief port 50 in the bonnet. Thecapacity of relief port 50 is much greater than. that of passage 36whereby the pressure within chamber 19 is quickly vented when port 50 isopened. Port 59 has a portion 50a that communicates with a passage 51 inthe bonnet. While the latter passage can open out into the atmosphere,it is ordinarily preferred to pass the fluid relieved from chamber 19into the conduit leading from the main relief valve. To do so, there isprovided a conduit 52 opening out into discharge port 13 and having itsother end opening into passage 51. The end of conduit 52. adjacent thebonnet has an enlarged portion 53 bearing seal rings 54 and 55 in sealedrelationship with the casing and the bonnet, respectively.

The pilot valve resides in an enlarged portion of port 59 formed incoupling nut 66 which is threadedly attached to bonnet 16. In order toprovide proper communication between passages 50a and 51, there isprovided in the face of coupling nut next to the bonnet an annularpassage 61 sealed on each side by annular seal rings 62 and 63. In thismanner passage 50a will always be in communication with passage 51irrespective of the rotational position of coupling nut 60.

The pilot valve includes a. fitting 64 threaded into coupling nut 60.This fitting carries at its upper end a housing 65 with an adjustingscrew nut 66 threadedly received therein. A cap 67 can be provided overthe top of the housing to protect the same and has one end inwardlythreaded to act as a stop nut for adjusting screw nut 66. A valve stem63 extends through nut 66 and is pressed downwardly by control spring 69which engages guide 70 formed on the valve stem. The other end of spring69 bears against a thrust bearing abutting one end of nut 66 andcomprising a spring retaining ring 72 which forms one of the races forbearing 73a. In this manner nut 66 can be screwed into and out ofhousing 65 to adjust the tension of spring 69 without applying anysubstantial rotative stress thereto. The lower end of stem 68 passesthrough a reduced diameter opening in the lower end of housing 65 and issealed therefrom by suitable sliding seal such as O-ring 73 residing ina peripheral groove formed in the housing. A passage 69a vents thehousing above O-n'ng 73 to the atmosphere.

Fitting 64 has a laterally extending opening 74 providing a passagebetween the interior of the fitting and passage 50a via peripheralgroove 75. Flow through passage 74 is controlled by a relief valvepiston 76 seating on a seat formed by boss 77 on the end of the seatmember 78. The latter is maintained in a fixed position in the fittingby a shoulder 79 hearing thereagainst. An O-ring seal 80 is providedbetween the seat member and the fitting. Extending through the seatmember is a passage 31 which discharges centrally of boss 77. Disposedaround the boss is O-ring 82 maintained in place by a washer 83 carriedby piston 76. With such construction, an effective seal is providedbetween passage 81 and piston 76, when the latter is seated on the boss,to prevent fluid escaping from the passage.

This arrangement is such that the pressure within chamber 19 actsthrough passage 81 to urge piston 76 from its seat on the boss. Thetension of spring 69 acts against such pressure through valve stem 68and such tension is adjustable by nut 66 to a magnitude that when apredetermined pressure exists in the system protected and in chamber 19,piston 76 will be forced upwardly to remove it from its seat on member78. Since passage 81 has a much greater flow capacity than passage 36 inbolt 34, the unseating of piston 76 will cause the pressure in chamber19 to rapidly decrease so that piston 22 is urged toward the bonnet bythe pressure in the system protected acting against valve member 21. Itis to be noted that when piston 76 has been cracked from its seat theentire cross-sectional area thereof will then be exposed to the pressureof the fluid from passage 81 causing the piston to snap quickly awayfrom member 78. When valve member 21 'has been unseated and piston 22pushed towards bonnet 16, bleed passages 33 and 36 are closed by bolt 34being pushed into bushing 35 so that the end ports of passage 36 aresealed. As a result the pressure within chamber 19 will not build upagain so that the relief valve will remain open until the pressurewithin such chamber is built up by a supplementary means. Such means cancomprise a low pressure pilot valve shown generally as numeral 90.

It is a feature of the pilot valve shown in Fig. 2 that any backpressure acting through passage a against the lower end of piston 76 isprevented from aiding the fluid pressure within chamber 19 in unseatingthe piston from its seat to thereby open the relief valve at a pressurebelow its set pressure. Provision against such contingency is effectedby providing a means for applying any back pressure in passage 50a tothe other side of the piston '76. Thus, passage 91 is provided toconnect with the upper end of piston 76 so that the back pressure willexert substantially no net force tending to move the piston and will notcause an opening thereof by acting solely on the lower face of thepiston. In a preferred embodiment, the net effective pressure areacontained within sealing means 73 disposed around the valve stem is madesubstantially equal to the net effective pressure area contained withinseating means 82 disposed around boss 77 so that althrough the efiectivepressure area of the upper end of piston 76 may be larger than that ofits lower end due to boss 77, this difference will be canceled out bythe back pressure acting to decrease the force of spring 69 by an amountequivalent to that which could have been applied by the back pressureover the area contained within sealing means 82. p

In order to aid the sealing effectiveness of O-rings 92 and 93 containedon piston 76, there is provided a passage 94 communicating with aperipheral groove 95 between the O-rings and the upper end of the pistonso that any pressure which may accumulate between the O-rings will bebled off.

The low pressure pilot valve 90 controls a passage in valve bonnet 16which in turn may communicate with the pressure to be controlled at anydesirable point as, for instance, by fitting 101, conduit 102, andpassage 103. Conduit 102 can have an enlargement 104 provided with sealrings 105 to elfect a communication between passage 103 and the conduit.

The low pressure pilot means includes generally a spring pressed plungerpiston 106, a valve element 107 and an adjustable spring biased valvestem and housing therefor of similar structure and function as thecorresponding parts of pilot valve 45, with the single exception that itis not necessary to provide a seal 73 around the valve stem.

Piston 106 is exposed to the pressure from passage 103 so that the forceof the pressure acting against thepiston opposes that of spring 108.Thus, when the pressure within passage 103, which it will be rememberedis the pressure within the system protected, falls below a predeterminedvalue, spring 108 will move piston 106 downwardly to unseat valve member107.

The detail construction of the low pressure pilot means includes afitting 109 threadedly received in a bushing nut 110 and sealed theretoby means of O-rings 111 disposed on each side of portion 100a of passage100. Bushing nut 110 has an annular passage 112 in the face thereofadjacent to the valve bonnet which is sealed on either side by O-rings.In this manner, communication between passage 100a and passage 103 isassured irrespective of the rotational position of bushing nut 110'.

Fitting 109 has a housing 115 threaded thereto, which has an adjustablecompression nut locked in an adjusted position by a cap 116 in a mannersimilar to that described for valve 45.

Piston 106 is engaged by valve stem 119 and has a slidable seal such as'O-rin'gs with an interior bore of the fitting. Valve member 107 isdependent from piston 106 and seats against an inwardly extendingshoulder member 126 which is borne by fitting 109.

In order to balance out the effect of the pressure of the fluid in thesystem being protected on the area enclosed within the seated valvemember 107 so that such pressure can effectively act solely on thepressure area of piston 106, there is provided a piston or plunger 127forming an extension of valve member 107 on its end downstream of itsseat and reciprocally received in a corresponding cylindrical portion128 of member 129 and maintained in sealed relationship therewith by asealing element such as 'O-ring seal 130. The effective pressure area ofplunger 127 is preferably made equal to the eflfective pressure of valveelement 107 across its seat with shoulder 126.

Communicating with cylinder 128 is a passage 131 having laterallyextending ports opening into the pressure fluid chamber 132. Alsocommunicating with this chamber is a restricted flow passage 133 betweensuch chamber and an annular groove 134 which opens out onto passage100a. In this manner, the force exerted by the pressure fluid in chamber132 on the valve element 107 over the effective pressure area of itsseat is counterbalanced by a fluid under the same pressure actingthrough passage 131 against plunger 127 so that the pressure of thefluid in the system protected does not have any net actuating force onvalve element 107 across its seat.

Member 129 is provided with a fluid passage 135 downstream of the seatof valve member 107 and this passage in turn communicates by an annularpassage 136, passage 137, and bore 138 with passage 100 into chamber 19.

Passage 137 is provided as a restricted flow area to permit manualtesting of the relief valve. Thus, when the system being protected istemporarily at a pressure below that at which a low pressure valve isset to open and it is desired to test the relief valve to determinewhether it is operating properly, valve stem 68 of the high pressurepilot valve can be lifted manually to permit the pressure in chamber 19to be vented in order to unseat valve member 21 from its seat 20. Withrestricted passage 137, the repressuring fluid from passage 103 cannotflow into chamber 19 at a rate sufliciently high to prevent piston 22from rising therein. Hence the operation of the relief valve can besatisfactorily tested even though the pressure in the system beingprotected is below the set pressure of the low pressure pilot valve.

Operation of this low pressure pilot means is believed apparent. Withvalve element 107 seated, the pressure from the system protected isactive against the area of piston 106 thereby urging it upwardly againstspring 108. It is to be noted that the pressure acting against the valvemember over the area seated within shoulder 126 is effectivelycounter-balanced by the same pressure acting in an opposite directionagainst the end of plunger 127 which is of equal area of that containedin the valve seat. Therefore, the seating and unseating of valve element107 is independent of the pressure existing in the chamber 19 and hencein the chamber below the valve seat. 'When the force due to the pressureacting against the end of piston 106 falls below that of spring 108, thespring forces piston 106 downwardly to unseat the valve element. Plunger127 is still exposed to the same pressure as exists in chamber 132 sothat the unseating of the valve element does not disturb the balance offorces due to unseating of the valve. When the pressure within thepressure system protected rises above a predetermined value, valveelement 107 will be reseated.

It is believed that the operation of the various parts of the reliefvalve of this invention is apparent from the foregoing description,however, a brief description of the operation of the relief valve as awhole will be given to ensure a complete disclosure of the invention.

The relief valve can be mounted upon any pressure system to be protectedand is particularly adapted to be mounted to discharge with one or moreother relief valves into a common discharge header without having theback pressure affected by opening of the other relief valves causingirregularities in the operation of the instant valve. When so mountedthe pressure of the system protected is active against the area of valvemember 21 which is enclosed within seat 20 to urge the valve membertowards an unseated position. However, this same pressure bleeds throughpassages 33 and 36 into chamber 19 and is active against the relativelylarger area of piston 22 in a direction to seat valve member 21.

As long as the pressure within the system protected is below theselected safe maximum, the relief valve member 21 will remain closed.However, when this pressure exceeds the selected maximum, the highpressure pilot valve 45 is opened and quickly effects a reduction in thepressure within chamber 19 so that the pressure acting against the valvemember 21 causes the valve member to rise against the forces actingagainst piston 22 in the opposing direction. When the valve member is inopen position, the bleed port opening from passage 36 is closed so thatthe pressure within chamber 19 remains relieved until the pressurewithin the system protected falls below a predetermined lower limit.When a this occurs the low pressure pilot valve 90 is opened to supplypressure to chamber 19. This pressure fluid is substantially the same asthat acting against valve member 21 to hold it open and, being effectiveacross a greater area than the area of valve member 21, the valve memberis forced to closed position. As soon as the pressure within the systemprotected builds up above the lower limit, low pressure pilot valve 90will close. It will be appreciated, however, that any time the pressurewithin the system protected falls below the predetermined lower limit sothat the pilot valve 90 opens, this will not have any effect upon therelief valve 21 but that it will remain closed.

When a back pressure is applied through outlet port 18 to the reliefvalve of this invention, the latter will not be opened when such backpressure exceeds pressure of the system being protected because flange24 prevents its access to piston 22. Thus, when a plurality of reliefvalves are connected to a plurality of pressure systems to be protectedas shown in Fig. 3, relief of pressure from any one of the systems willnot cause the relief valve of any of the other systems which may have aset pressure lower than the back pressure effected by such relief valve,to open. For example, if the system protected comprises compressors andone of the compressors is shut down so that the pressure throughout therelief valve protecting that system falls to atmospheric pressure, theopening of one or more of the other relief valves operating on systemswhich have not been shut down will not cause the relief valve of thesystem which has been shut down to fly into an open position due to backpressure applied thereto. Specifically, if the valve shown in Fig. 1 ismounted on a system whose pressure is substantially atmospheric and apressure be applied through outlet port 18, such pressure will beprevented from acting against the lower side of piston 22 by inwardlyextending flange 24 and the pressure on the lower side of piston 22 willremain atmospheric, irrespective of the magnitude of the pressureexerted through outlet port 18, due to venting of chamber 27 throughport 28. Hence it is believed to be apparent that a back pressureapplied to the valve will not cause it to open but will, on thecontrary, act to urge valve member 21 even more firmly into its seat 20.

With the construction of the relief valve of this invention similar tothat illustrated in the drawings, it is believed to be apparent that thevalve is particularly adapted to be used in systems having a very closeset relief and closing pressures. Thus, when the valve of this inventionis employed to protect a pipe line having a safe maximum pressure limitof 700 p. s. i. it can be set to open at 690 p. s. i. and to close at680 p. s. i. thereby providing a very narrow operating range withresultant economies in operation. The advantage of this close setoperation will be even more apparent when it is remembered that manyother types of relief valves must be set to open at a pressure which issubstantially below, say 30 to 50 p. s. i. in the example given above,the safe maximum pressure of the line in order to assure that the linewill be adequately protected. Thus, the operating pressure of a line,when using the instant valves, may be as much as 40 p. s. i. above thatpermissible with other types of relief valves and still afford the samedegree of protection.

Another advantage of the valve of this invention is that there is nonecessity for permitting escape of fluids across the main valve seatinto a so-called huddling chamber as in many other types of valves inorder to effect an operation thereof. Accordingly, there is noopportunity for such gases or fluids to erode the seat of the valve bytheir constant passage therethrough.

From the foregoing it will be seen that this invention is one welladapted to attain all of the ends and objects hereinabove set forth,together with other advantages which are obvious and which are inherentto the structure.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of theclaims.

As many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth or shown in the accompanying drawings is to beinterpreted as illustrative and not in a limiting sense.

This invention having been described, what is claimed 1. In a system tobe protected which includes a plurality of conduits connected with acommon header, a relief valve in each conduit for venting the fluid insaid conduit into the header, each of said relief valves comprising, avalve body having an inlet connecting with a conduit and an outletconnecting with the common header with a passage between the inlet andoutlet, a seat in the passage for controlling flow to and from theoutlet, a valve member controlling the seat and mounted to be urged byinlet pressure in an unseating direction and by outlet pressure in aseating direction, a pressure actuator for the valve member and carriedby the body, said actuator including means providing a pressureresponsive member of larger effective area than that of the valve memberwithin said seat and having a power transmitting connection with thevalve member, so that the pressure responsive and valve members movetogether as a unit, restriction means for supplying fluid under pressurefrom said inlet to one side of the pressure responsive member to opposemovement of the valve membet in an unseating direction, means shieldingthe opposite side of said pressure responsive member from the valveinlet, outlet and passage, and exposing it to atmosphere so that uponthe pressure from an inlet of one relief valve in said system beingdecreased to atmospheric and another relief valve in said system openingto relieve pressure into said header, the resultant back pressure on thevalve member of said one relief valve urges is to its seat and isprevented from acting upon the larger area of said opposite side of saidpressure responsive member, and means responsive to the pressure on thepressure side of the pressure responsive member for relieving the lastmentioned pressure.

2. A pressure responsive valve which comprises, in combination, a valvecasing having an inlet and an outlet port, a valve member therebetweento seat against pressure from said inlet port, a piston of largereflective pressure area than said valve member disposed in said casingto form therewith a pressure chamber on one side of said piston, powertransmitting connection means between said piston and said valve member,means for bleeding pressure fluid to be controlled into said pressurechamber, a snap action high pressure pilot means for relieving pressurefrom said pressure chamber upon occurrence of a predetermined maximumpressure in the system protected by said valve; said pilot meansincluding a body providing a cylinder, a first conduit connectingbetween the pressure chamber and an end of the cylinder and having aseat about its connection with said cylinder, a second conduitconnecting between the cylinder and the outlet port of the valve casing,a piston movable within the cylinder between a position in which apressure responsive face thereof is seated upon the first conduit seatto close the same and another position in which said face is spaced fromthe seat to communicate said first and second conduits, means urging thepiston to said seated position but responsive to a predeterminedpressure within the pressure chamber and acting upon the portion of saidpressure responsive face within said seat to permit said face to beunseated, the connection of said second conduit with the cylinder beinglocated to communicate said second conduit with the piston faceexteriorly of that portion within the seat; and conduit means in saidbody connecting the second conduit with the pressure responsive face ofthe piston opposite to the first-mentioned face, a flange extendinginwardly of said casing to seal 011 from said outlet port the side ofsaid piston opposite said pressure chamber to form a chamber betweensaid flange and said opposite side of the piston, and means venting saidlast-mentioned chamber.

3. A valve of the character defined in claim 2, wherein the pistonurging means includes an actuator sealably received through an openingfrom the end of the cylinder opposite the seat, and the effectivepressure area across said opening is substantially equal to thatcontained within the seat.

References Cited in the file of this patent UNITED STATES PATENTS134,435 Mayer Dec. 31, 1872 135,437 Mayer Feb. 4, 1873 919,300 AndersonApr. 27, 1909 1,229,726 Ebling June 12, 1917 1,611,079 Roschanek Dec.14, 1926 1,627,628 Anderson May 10, 1927 1,805,702 Mastenbrook May 19,1931 2,500,888 Wilhelm Mar. 14, 1950 2,574,414 Ragland Nov. 6, 19512,576,541 Schmitt Nov. 27, 1951 2,619,109 Garrett Nov. 25, 1952

